Technologies relating to contactless actuators as disclosed in U.S. Pat. No. 4,195,277 are known in the state of the art. Such an actuator relates to the utilisation of a moving armature composed of a single-pole magnet. The moving assembly faces a stator structure composed of a “C”-shaped circuit containing a coil. Such structure has the advantage of a constant variation of the magnetic flux caused by the magnet through the coil, during the motion of the magnet. Such flux gradient per motion unit is called the force factor of the actuator; when multiplied by the magneto-motive force of the coil, it makes it possible to obtain the force generated by said actuator. Such force is then constant for a constant force factor and a given current. However the structure provided is not adapted to actuators which must have a high force factor because only one pole is used on the moving magnet and 2 poles are used on the stator.
3-stator pole structures can thus be found in the state of the art and more particularly in the applicant's patent WO90/16109 A1, which make it possible to enhance the force factor by using two poles on the moving magnet. If it is desired to create an actuator having a high force per electric power unit, a high magneto-motive force of the coil is required in addition to a high force factor. This requires a large dimensioned coil. Such structure thus has the drawback of very large axial overall dimensions since three poles are used on the stators, to which twice the height of the coils must be added. According to the teachings of this patent, the travel of such an actuator is then smaller than half the difference between the total height of the magnetic circuit including the coil and the total height of the magnetized parts (2·YA). Thus, for instance, for a total height of the magnetic circuit of 52 mm and a total height of the magnetized parts of 37 mm, such an actuator would have a travel of 7.5 mm only enabling it to produce a constant force over this travel.
The applicant's patent EP00607354B1 is also known, which proposes to remedy such a problem of overall dimensions by using a housing provided in the stator part, wherein the coil is embedded. The axial overall dimensions of the structure are then much more reduced since it is equivalent to the added height of the three stator poles which twice the distance between the central pole and the lateral pole is added to, with this distance being reduced, this time, to half the air gap E, which corresponds to the sum of the magnet thickness and the mechanical clearance between the magnet and the surface of the stator. According to the teachings of this patent, the central pole must have a height YC equal to the travel increased by the distance E, with the width of the lateral poles YL being greater than the width of the central pole. Thus, for a height of 52 mm, the teachings of this patent lead us to an actuator having a maximum travel of 14 mm, which is a real improvement with respect to patent WO90/16109 A1. The direct compensation therefor is that the result is a proximity between the central pole and the lateral poles, which favours the leakages of the magnetic flux between the stator poles. With a view to reducing the leakage flux between the poles, the specialists in the art then react by chamfering the poles, which is also disclosed in patent EP00607354B1.
Such actuators are for instance found in automotive applications wherein the constant characteristic of the force over the entire travel as well as the proportionality of force with current are appreciated, i.e. when it is desired to regulate the positioning of a valve to adjust the flow of a fluid (for instance exhaust gas recirculation valves) or to regulate the positioning of a pedal to make the latter active and able to transmit information to the driver. The problem of overall dimensions is particularly important in such small spaces since each millimeter saved is precious to enable the positioning of the actuator. However, although the last mentioned actuator has the advantage of providing dimensioning rules for a compact actuator in order to produce a constant force over the travel, in nominal rating, it has the drawback of not being adapted to the problems entailed by enlarged travel compact actuators able to produce a constant force over the entirety of its travel, and more particularly for high saturation ratings.
The application for U.S. Patent Publication No. 2008/0220930, which discloses a linear actuator comprising tubular permanent magnets positioned around an armature core for an axial motion in a tubular stator upon exciting concentric coils connected to the armature, is also known in the state of the art. The stator has parts radially extending towards the inside of the coils and one towards the other under each coil, which defines a space between the coil and the armature. The rings have a substantially radially magnetized structure and the coils are configured for the input of single-phase power. Two pairs of spaced crowns are positioned on the core, wherein the external axial length of the rings is equal to half the axial length of the rings it is provided with. This solution has the drawback of poor overall dimensions and force feature.